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Friday, 18 October 2013

Unexploded device? The vCJD timebomb

Principles of Molecular Virology, Chapter 8 (Sub-Viral Agents), discusses entities such as viroids and prions responsible for diseases such as scrapie, BSE and CJD.

Creutzfeldt–Jakob disease (CJD) is a human prion disease in which a protein found in healthy cells is corrupted into a form which causes the cells to die. CJD occurs sporadically at low rates in humans but in the 1980s a new form of the disease was identified - variant Creutzfeldt–Jakob disease (vCJD). The difference between the two is that vCJD is more deadly than CJD - it has a shorter time course and kills more rapidly - but chillingly, vCJD (unlike CJD) is infectious and can be "caught". Even more worryingly, vCJD appears to be the human form of "mad cow disease" (bovine spongiform encephalopathy or BSE).

In the 1980s and early 1990s, millions of people were exposed to infection through eating beef (and other food products containing gelatine, etc) from cows with BSE. Thankfully, most of the people who were exposed to the infectious form of the prion did not die. To date, here have been 177 deaths from vCJD in the UK. Most of these occurred in the late 90s and early 2000s and there has been only one death in the last two years. But prion diseases take a long time to incubate and show themselves. The closest parallel to BSE is a disease called kuru, which was a prion disease transmitted from human to human. In kuru, the average incubation period between infection and death was 14 years, but some cases have been recorded which occurred 40 years or more after infection. So what will be the ultimate fate of all those people who were exposed to the BSE prion tow or three decades ago. Do they become carriers, silently incubating the prion within them, or will they ultimately become sick and die?

Research just published (Prevalent abnormal prion protein in human appendixes after bovine spongiform encephalopathy epizootic: large scale survey. BMJ 2013; 347: f5675) examined more than 32,000 samples of human tissue removed during appendix operations carried out between 2000 and 2012 at 41 hospitals in the UK. The results indicate that one in 2,000 people in the UK - over 25,000 people - is a carrier of vCJD, for which there is currently still no effective cure. This number is twice as high as previous estimates. Apart from whether these people will ultimately become ill, another worrying aspect of this statistic is whether these silent carriers can pass the infection on to others through blood transfusions. There is no reliable test to screen for vCJD carriers. In the UK, there have been three reported cases of vCJD associated with a blood transfusion - thankfully, a lower number than might have been expected if the carrier state made people highly infectious.

So many facts about human prion disease remain unknown, but what is clear is that decades after it started, mad cow disease has not gone away - the effects of the outbreak will rumble on for decades to come.